The present invention relates to a magnetoresistive effect element including a ferromagnetic substance a magnetizing direction of which is changed by an external magnetic field, a magnetic memory device for storing information by utilizing a change in the magnetizing direction, and a method of fabricating the same.
Conventionally, as a general-purpose memory used in information processing apparatus of a computer, a communication apparatus and the like, a volatile memory such as DRAM, SRAM or the like is used. The volatile memories need to refresh by incessantly supplying current for holding memory. Further, since all the information is lost when a power source is cut off, a nonvolatile memory needs to install as means for recording information other than the volatile memories and, for example, flash EEPROM, a magnetic hard disk device or the like is used therefor.
In the nonvolatile memories, high speed access poses an important problem in accordance with tendency of high speed information processing. Further, there is rapidly progressed development of information apparatus aiming at so-to-speak ubiquitous computing capable of carrying out information processing at anytime and at anywhere in accordance with rapid spreading the high performance portable information apparatus. Development of a nonvolatile memory applicable to high speed processing has strongly been requested as a key device constituting the core of developing such an information apparatus.
As a technology effective to speedup a nonvolatile memory, there is known a magnetic random access memory (hereinafter, referred to as MRAM) in which magnetic memory elements for storing information by a magnetizing direction along an easy magnetization access of a ferromagnetic layer are aligned in a matrix shape. In MRAM, information is stored by utilizing a combination of magnetizing directions in two ferromagnetic substances. Meanwhile, stored information is detected by a change of resistance (that is, change of current or voltage) produced by a case in which the magnetizing directions are in parallel and same direction with each other relative to a certain direction constituting a reference and a case in which the magnetizing directions are in parallel and opposite with each other.
An MRAM which has currently been reduced into practice utilizes a giant magneto-resistive (GMR) effect. The MRAM utilizing a GMR element achieving the GMR effect described in U.S. Pat. No. 5,343,422 has been known. The GMR effect is a phenomenon in which a resistance value becomes a minimum value when magnetizing directions in two parallel magnetic layers along a direction of an easy magnetization axis are in parallel and same direction with each other, and becomes a maximum value when the magnetizing directions are in parallel and opposit direction with each other. As MRAM using a GMR element, there are a coercive force difference type (Pseudo spin valve type) and an exchange bias type (spin valve type). In the case of MRAM of the coercive force difference type, the GMR element includes two ferromagnetic layers and a nonmagnetic layer interposed therebetween and information is written and read by utilizing a difference between the coercive forces of the two ferromagnetic substances. Here, a resistance change rate when the GMR element is constructed by a constitution of, for example, “nickel iron alloy (NiFe)/copper (Cu)/cobalt (Co)” is a small value of about 6 through 8%. Meanwhile, in the case of MRAM of the exchange bias type, the GMR element includes a fixed layer fixed with the magnetizing direction by antiferromagnetic coupling with an antiferromagentic layer, a free layer the magnetizing direction of which is changed by an external magnetic field and a nonmagnetic layer interposed therebetween for writing and reading information by utilizing a difference between the magnetizing directions of the fixed layer and the free layer. A resistance change rate when the GMR element is constructed by a constitution of, for example, “platinum manganese (PtMn)/cobalt iron (CoFe)/copper (Cu)/CoFe” is about 10% which indicates a value larger than that of the coercive force difference type. However, this value is insufficient for achieving a further increase in the storing speed or a further increase in the access speed.
In order to resolve the points, there has been proposed MRAM having a TMR element utilizing a tunnel magnetoresistive effect (hereinafter, referred to as TMR effect). The TMR effect is an effect in which by a relative angle in magnetizing direction between two ferromagnetic layers interposing an extremely thin insulating layer (tunnel barrier layer), tunnel current flowing to pass the insulating layer is changed. When the magnetizing directions of the two ferromagnetic layers are in parallel and same direction with each other, a resistance value is minimized and when the magnetizing directions are in parallel and opposite direction with each other, the resistance value is maximized. In the case of MRAM utilizing the TMR effect, when the TMR element is constructed by a constitution of “CoFe/aluminum oxide/CoFe”, the resistance change rate is as high as about 40%, further, also the resistance value is large and therefore, matching in the case of being combined with a semiconductor device of MOSFET or the like is easy to take. Therefore, an output higher than that of MRAM having the GMR element is easy to achieve and an increase in the storage capacitance or the access speed is expected. In the case of MRAM utilizing the TMR effect, there is known a method of storing information by changing a magnetizing direction of a magnetic film of a TMR element to a predetermined direction by a current magnetic field generated by making current flow in a lead wire. As a method of reading stored information, there is known a method of detecting a change in the resistance of the TMR element by making current flow in a direction orthogonal to the tunnel barrier layer. Further, with regard to MRAM using the TMR effect, there is a description in U.S. Pat. No. 5,629,922 or JP-A-9-919149 or the like.
As described above, according to MRAM utilizing the TMR effect, output formation higher than that of MRAM utilizing the GMR effect can be achieved. However, even in the case of MRAM utilizing the TMR element showing the resistance change rate of about 40%, output voltage is about several tens mV and therefore, the output voltage is insufficient for realizing a magnetic memory device having a higher density.
FIG. 32 is a plane view for explaining a constitution of a conventional magnetic memory device utilizing a TMR effect and FIG. 33 shows a sectional constitution of an essential portion of the magnetic memory device in correspondence with FIG. 32. Read and write word lines 112 and 106 and a bit line 105 are intersected orthogonally and a TMR element 120 comprising a first magnetic layer 102, a tunnel barrier layer 103 and a second magnetic layer 104 is arranged to be interposed by the orthogonally intersected portion. In the case of MRM of a type in which the write bit line 105 and the write word line 106 are orthogonal to each other, a magnetizing direction in the second magnetic layer 104 constituting a free layer cannot sufficiently be aligned and it is difficult to carry out sufficiently stable writing.
Further, in the case of MRAM utilizing the TMR effect, information is stored to each memory cell by changing a magnetizing direction of a magnetic film by an induced magnetic field by current flowing in lead wires which are arranged orthogonal to each other, that is, a current magnetic field, however, the current magnetic field is an open (not confined to a specific region magnetically) magnetic field and therefore, not only the efficiency is low but also there is a concern of effecting adverse influence on a contiguous memory cell.
Further, in the case of achieving higher density formation of a magnetic memory device by highly integrating memory cells, miniaturization of a TMR element is indispensable, however, there is a concern of the following problem. That is, it seems that a counter magnetic file is increased by increasing an aspect ratio (thickness/width in laminated layer face direction) of each magnetic layer in the TMR element, a magnetic field intensity for changing the magnetizing direction of the free layer is increased and large write current is needed.